Adding hot memory to a virtual machine fails or the option is greyed out

Products

• VMware vSphere ESXi

Issue/Introduction

• When attempting to add hot memory to a virtual machine, the option is greyed out.
• Adding hot memory to a virtual machine fails.

Environment

• VMware vSphere ESXi 7.x
• VMware vSphere ESXi 8.x

Cause

Enabling Memory Hot Add forces the hypervisor to present a single interleaved virtual NUMA node (UMA) to the guest OS. This occurs because ESXi must maintain flexibility for future memory expansion, which prevents the static optimization required for vNUMA.

In vSphere 8.0 and later (Hardware Version 20+), CPU Hot Add has been improved to maintain vNUMA topology (using numa.allowHotadd = TRUE), but Memory Hot Add still completely deactivates vNUMA.

Resolution

Most common cause for this issue and the resolution steps are listed below:

1. Ensure the host has the required license: Hot Pluggable Virtual Hardware:
   1. Log in to the vSphere Client and select the ESX host.
   2. Go to Host > Configure > Licensing > License Details
   3. Select Licensed Features.
   4. Verify if you have Hot Pluggable Virtual Hardware.
      
      
2. Ensure the Virtual Machine has the option enabled:
   1. Log in to the vSphere Client.
   2. Right-click on the virtual machine.
   3. Navigate to Edit Settings > Options > Memory/CPU HotPlug.
   4. Select Enable Memory Hot Add for this Virtual Machine.

Note: It is equally important that the guest operating system supports the hot add feature. To determine if it does, see the Broadcom Hardware Compatibility List (choose Guest OS from the What are you looking for dropdown). The virtual hardware section under the Guest OS should read supported.

Following two scenario may also cause this issue as well:

• Memory Reservation: If the setting memoryReservationLockedToMax is set to true, hot-add will fail.
• Guest OS Mismatch: If the configured Guest OS in VM settings does not match the actual installed OS, the hot-add option may be greyed out or fail.

Additional Information

Note: Enabling this feature disables vNUMA, which can impact the performance of large virtual machines (typically those with 8 or more vCPUs).